Not at night...
...and it's not free, you paid for those panels and the running cost is the amount that you would otherwise gain from selling to the grid.
Don't forget electricity goes through massive interconnects. The surplus (if any) gets sold to other parts of the country and the excess demand gets covered from other sources.
And the total generation only covers 2/3 of the usage, so 1/3 is "imported".
And 60% of those 2/3 comes from burning gas.